1. Field of the Invention
The present invention relates to an image pickup apparatus having an expanded photographing region ranging from the visible wavelength region to the infrared wavelength region. In particular, the present invention relates to a visible light component ratio calculation method for calculating the ratio of the visible wavelength component contained in the light emitted from a light source for illuminating an object to be photographed and to an image pickup apparatus using such a visible light component ratio calculation method.
2. Description of the Related Art
There are prior arts such as the image pickup apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-224469 and the camera provided with a shooting mode switching function disclosed in Japanese Patent Application Laid-Open No. 2003-219254. These prior arts relate to a dual-purpose infrared/visible-light camera that is adapted to switch the shooting mode automatically and insert/retract an infrared cut filter automatically. According to these prior arts, the shooting mode is determined based on a brightness signal and a color signal obtained from an image-pickup element.
In the above mentioned prior arts, when the current shooting mode is color shooting mode, the shooting mode is determined based on whether a brightness signal Y is larger or smaller than a predetermined threshold value Y2. On the other hand, when the current shooting mode is black-and-white shooting mode, the shooting mode is determined based on whether the brightness signal Y is larger than or smaller than another predetermined threshold value Y1. Here, one threshold value Y1 is set to a value larger than the other threshold value Y2 (Y1>Y2). This condition is introduced in order to prevent hunting that may occur when the shooting mode is switched from black-and-white shooting mode to color shooting mode with a rise in the brightness signal caused by removal of the infrared filter.
Furthermore, when the current shooting mode is black-and-white shooting mode, color signals are also analyzed in addition to the brightness signal in determining the shooting mode. The analysis of the color signals are performed by obtaining two color ratios or the ratio of the red signal and the green signal and the ratio of the blue signal and the green signal. A determination is made as to whether the picked-up image signal is derived from near-infrared light or visible light based on whether the aforementioned two color ratios fall within a specified distribution range. Referring to sensitivity characteristics of the image pickup element for respective colors, in the wavelength region longer than approximately 800 nm, the sensitivity is substantially identical irrespective of colors. When the picked-up image signal is derived from near-infrared light, the aforementioned two color ratios fall within the specified distribution range. When it is determined that the picked-up image signal is derived from near-infrared light, the black-and-white shooting mode is not changed.
In the case of the camera provided with a shooting mode switching function disclosed in Japanese Patent Application Laid-Open No. 2003-219254, when a large field is shot, the color signals are averaged over the entire photographing screen, and it is not possible to extract a characteristic point. As a countermeasure, the photographing screen is divided into a plurality of blocks, and a determination is made as to whether the picked-up image signal is derived from near-infrared light or not for each block, and the number of the blocks in which the picked-up image is derived from near-infrared light is counted. According to the disclosure of this patent document, a determination is made as to whether the picked-up image signal is derived from visible light or near-infrared light based on the count number thus obtained.
In conventional cameras, under a mixed light source such as incandescent lighting or sunlight that contains both visible components and infrared components, the brightness in the picture frame in shooting becomes unstable at the time when the infrared cut filter is inserted or removed. For example, in the case where a green or blue object is shot under incandescent lightening, the lighting may be misinterpreted as infrared lighting even if the brightness of the subject is sufficiently high, and the photographing mode is not switched to color shooting mode in some cases. In other cases, hunting may occur.